1. Field of the Invention
The present invention relates to an imaging apparatus, a control method thereof and a program, and more particularly to an apparatus for acquiring an X-ray image and for acquiring a digital image.
2. Description of the Related Art
Technology for irradiating radiation typified by X-rays onto a substance, and measuring and imaging an intensity distribution of the radiation attenuated as a result of being transmitted through the substance has brought about the development of medical technology. Since the discovery of X-rays, a technique for imaging the intensity distribution has been adopted that involve making and developing a latent image on silver-halide film after converting the X-ray intensity distribution to visible light using phosphors. In recent years, a technique using so-called imaging plates that involves digitizing an X-ray image using photostimulable phosphors, by exciting with laser light and reading out a latent image formed as the distribution of stored energy in the photostimulable phosphors that results from X-ray irradiation has become popular. Further, large format solid-state image sensors, or so-called flat panel detectors, that can cover the whole body have been developed through advances in semiconductor technology, enabling efficient diagnosis to be carried out by digitizing X-ray images directly without making a latent image.
Meanwhile, it has also become possible to image fluorescence resulting from attenuated X-rays using a high-sensitivity image sensor typified by photomultiplier tube (image intensifier), and observe the dynamic state within the body, with this technology now in increasingly common usage. The sensitivity of recent flat panel detectors is comparable with these image intensifiers, with it now being possible to X-ray the dynamic state over a wide range of regions of the body.
A flat panel detector converts the intensity distribution of X-rays that have passed through an object into the light intensity distribution of a scintillator or a two-dimensional distribution of electron density resulting from free electron excitation of semiconductors. In order to extract image information as a one-dimensional electrical signal, sequential scanning is performed using a transistor called a TFT (Thin Film Transistor) that is implemented by being printed in high density in a two-dimensional planar state using semiconductor manufacturing technology.
Principles of Flat Panel Detectors
FIG. 11 shows an example of a typical configuration of a flat panel detector for converting the light intensity distribution of a scintillator into an electrical signal. In the example in FIG. 11, the flat panel detector is constituted by adhering a scintillator 201 and a planar image sensor 202 or by directly depositing the scintillator onto the image sensor. When X-rays having a spatial intensity distribution reach the flat panel detector from the direction of the arrow in FIG. 11, the scintillator emits light in accordance with the X-ray intensity distribution, and image information is extracted as an electrical signal by the image sensor.
FIG. 12 represents the internal configuration and peripheral circuitry of the common image sensor 202, and FIG. 13 schematically shows the configuration of a conventional X-ray imaging apparatus that incorporates the image sensor 202.
In FIG. 12, the block enclosed by a broken line 101 is a single pixel. The pixel 101 has a photodiode 102 and a TFT 103, which is a field-effect transistor. Normally, these pixels are arranged grid-like in a planar state at an interval of around 0.1 to 0.2 mm. The TFT has a gate signal line 108 and a source signal line 109 similarly to a normal field-effect transistor.
The gate signal line 108 arranged on a two-dimensional plane is connected to another gate signal line in the latitudinal direction, and to a single output of a shift register 104 (111), as shown in FIG. 12. Switching control of the transistor gate is thereby performed. A similar connection is applied to all gate signal lines of the pixels aligned in the latitudinal direction, and the gate signal lines are connected to the output of the shift register 104. Control of the shift register 104 is performed by a row selection control unit 142 as shown in FIG. 13. Specifically, gate signals are sequentially selected in the shift register 104, as a result of a clock pulse input (105) to the shift register 104.
On the other hand, the source signal lines, which are the output of the TFTs, are all connected in the longitudinal direction such as shown in FIG. 12, and signals are input to a multiplexer 106 via sample and hold circuits (S/H circuits) 113. Hereinafter, the S/H circuits 113 and the multiplexer 106 will be collectively called a sample/hold-multiplexer circuit (S/H-MPX) 110.
This connection is applied to all source signal lines of the TFTs of pixels arranged in the longitudinal direction. Control of the sample/hold-multiplexer circuit 110 is performed by a column selection control unit 141 such as shown in FIG. 13. Specifically, control for sequentially selecting input signals in the multiplexer 106 after the signals have been sampled and held in the S/H circuits 113 is performed. The output of the multiplexer 106 is then sequentially input to an amplifier (AMP) 152, as shown in FIG. 13.
Planar scanning is completed by repeating the row selection control and the column selection control. The output of the amplifier 152 is input to an analog-to-digital converter 121 in order to be converted to digital values as image information, and a digital value sequence serving as an image signal is output.
The system, enclosed by a broken line 120 in FIG. 13, from input of the X-ray intensity distribution to output of the digital value sequence as image information is called an X-ray imaging system. The image sensor 202 used here is manufactured using semiconductor manufacturing technology, with difficulties being encountered in uniformly manufacturing the photoelectric conversion characteristics or capacitance characteristics of the pixels 101. For this reason, the respective characteristics of individual pixels generally differ. Also, difficulties are additionally encountered in ensuring the uniform accuracy of capacitances in the sample and holds and the uniform accuracy of the plurality of amplifiers that exist. Consequently, the electrical signals thus obtained contain the conversion characteristics of conversion from the X-ray intensity per pixel to the digital value sequence serving as an image signal, and cannot be directly handled as image information representing the X-ray intensity distribution.
Offset Correction
Generally, image information that is proportionate to X-ray intensity is acquired by correcting the conversion characteristics of each pixel in the X-ray imaging system, based on image signals containing the characteristics of each pixel. The principles of this image signal correction (offset correction) will therefore be outlined.
For example, when the X-ray intensity incident on a given pixel is defined as X, and the corresponding electrical signal output is defined as Y, the relation between X and Y can be represented with the following equation (1):Y=aX+b  (1)
Here, a is a coefficient representing the proportional relation between X-ray intensity and output, and is called a gain coefficient. Also, b is the signal level originally added to the signal, and is called an offset coefficient. Image signals need to be corrected because the gain coefficient (a) and the offset coefficient (b) differ for each pixel.
To correct the characteristics of each pixel, the above a and b are measured separately, and held as a gain correction value A and an offset correction value B. A signal equivalent to X is obtained by performing an inverse conversion of equation (1) using the gain correction value A and the offset correction value B. Note that the gain coefficient corresponds to the gain correction value, and that the offset coefficient corresponds to the offset correction value.
The offset correction value B can be derived by acquiring an image signal in a state where X-ray irradiation is not performed (X=0), and taking this image signal as B. Further, the gain correction value A can be calculated by performing X-ray irradiation at an intensity corresponding to X=1 uniformly over the flat panel detector to obtain an image signal, and subtracting B therefrom.
The X-ray intensity X can be corrected by performing an arithmetic operation such as equation (2) on the measured electrical signal output using A and B. That is, this can be represented with the following equation (2), by substituting the output image information.X=(Y−B)/A  (2)
FIG. 13 shows a common configuration for performing the above correction.
In FIG. 13, a block 122 is an offset correction value holding memory that stores offset correction values for all pixels. A block 123 is an offset correction unit that corrects offset by subtracting the offset correction values from image signals including the object that are normally output.
A block 124 is a gain correction value holding memory that holds gain correction values resulting from offset correction performed based on signals obtained by performing uniform X-ray irradiation. A gain correction unit 125 uses offset corrected signals output from the correction unit 123 to perform a division operation or a subtraction operation after performing log-transformation, thereby acquiring image information with respect to which the characteristics of each pixel of the sensor have been corrected.
A block 127 is a defective pixel value correction unit that corrects the pixel values of pixels in the image sensor that are not functioning (defective pixels). The defective pixel value correction unit 127 normally estimates and corrects defective pixel values using an averaging operation or the like based on the pixel values of neighboring non-defective pixels. The position of defective pixels is ascertained beforehand, and recorded for use by a defective pixel position holding memory 126. In FIG. 13, output resulting from the correction process is obtained with a signal 128 as a result, and a corrected image 129 is acquired.
FIGS. 14A and 14B show example changes in signal before and after correction, with FIG. 14A showing an example change in signal before and after correction in an ideal environment. In FIG. 14A, the horizontal axis represents the row position of the output image 129, and the vertical axis represents the pixel value. In FIG. 14A, a graph 132 shows pixel values before the correction process is performed, and is equivalent to data on a signal line 131 in FIG. 13.
A graph 133 in FIG. 14A shows offset correction values of the line 131 that are held in the offset correction value holding memory 122 in FIG. 13. A graph 134 plots the signals 128 of the rows in FIG. 13 on which sensor characteristic correction has been performed. The result represents the fact that an output image with smooth signal information is obtained by performing a sensor characteristic correction process for precisely representing the X-ray intensity distribution of input, despite the signal 132 fluctuating greatly under the influence of the sensor characteristics.
Note that in terms of configurations for detecting defective pixels, a configuration that performs defective pixel correction using whichever of vertical addition or horizontal addition results in an increase in the number of effective pixels is known (see Japanese Patent Laid-Open No. 2009-049527).
The offset coefficient and the gain coefficient, being strongly dependent on the semiconductor characteristics, typically change under the influence of environmental change (temperature, humidity) and degradation over time. Naturally, the sensor characteristic correction process will not be performed normally, in the case where the offset correction value or gain correction value held beforehand differ from the offset coefficient or gain coefficient when X-ray imaging is actually performed. For this reason, variations in sensor characteristics per pixel remain in the output image.
Normally, it is necessary to regularly acquire and correct the offset correction values or gain correction values, and rewrite the content of the offset correction value holding memory 122 or the gain correction value holding memory 124 as provision for this change. This processing is called calibration.
FIG. 14B shows an example change in signal before and after correction in the case where the offset coefficient of the actual sensor output changes under the influence of temperature fluctuation or the like. A graph 135 shows the change in the actual offset coefficient of the sensor output; that is, the result of performing sensor characteristic correction using the existing offset correction value 133 recorded in the offset correction value holding memory 122. A graph 136 plots the pixel values before the correction process is performed, and represents the data on the signal line 131 in FIG. 13. A graph 137 is the result of performing the correction process. As shown by the graph 137, in an environment in which the influence of environmental change, degradation over time or the like exists, a smooth result will not obtained without correction being appropriately performed.
This phenomenon will now be described. An output Y′ obtained after a change in temperature is represented as follows, focusing on a single given pixel, where b′ is the offset coefficient resulting from temperature change:Y′=aX+b′  (3)
When Y′ is corrected with the offset correction value B, an output X′ obtained after correction is represented by the following equation:
                                                                                             X                  ′                                =                                                      (                                                                  Y                        ′                                            -                      B                                        )                                    /                  A                                                                                                        =                                                      (                                          AX                      +                                              b                        ′                                            -                      B                                        )                                    /                  A                                                                                                                          ∴                                      X                    ′                                                  =                                  X                  +                                                            (                                                                        b                          ′                                                -                        B                                            )                                        /                    A                                                                                                            (          4          )                    
Here, since b′≠B due to temperature fluctuation, the value of the calculated signal X′ will differ from the originally intended X, with the second component ((b′−B)/A) remaining. Hereinafter, this component will be called a correction error. Although its appearance is unpredictable, the correction error often manifests in the display image as a fixed pattern originating in the manufacturing process or configuration of flat panel detectors.
With a conventional configuration, recalibration that involves reacquiring offset correction values and gain correction values must be performed after this correction error has manifested as an unpredictable fixed pattern on the image, at the stage at which the image observer feels that something is not quite right. That is, in order to determine the presence of a correction error and judge whether calibration needs to be performed, the observer needed to judge images not originally part of the object through observation. Here, it is difficult to recover an image once a correction error has manifested itself.